DESCRIPTION (adapted from investigator's abstract); The long term goal of this research is to understand the molecular mechanisms by which Rhizobium meliloti invades the nodules it elicits and establishes a productive symbiosis. Either of two R. meliloti exopolysaccharides, succinoglycan of EPS II, are required for nodule invasion and possibly for nodule development. More than twenty exo genes affecting succinoglycan biosynthesis have been identified and experiments will be carried out to refine the model of succinoglycan biosynthesis and to test hypothesis explored as well as the possible symbiotic importance of the systems responsible for their respective secretion. The regulation of succinoglycan production will be studied and further analysis of the ExoS/ChvI two component regulatory system will be carried out. Since extremely small amounts of a specific size class of EPS II oligosaccharides are sufficient for the symbiotic role of EPS II, the mechanisms responsible for EPS II synthesis and the control of its molecular weight distribution will be investigated. Green fluorescent protein will continue to be used to define the basis of the symbiotic deficiency caused by an inability to produce a symbiotically active exopolysaccharide and studies will be initiated to identify the putative plant receptors that sense the EPS II oligosaccharides. These studies will yield insights into the general mechanism by which bacteria synthesize and regulate surface polysaccharides and also into roles that bacterial surface polysaccharides can play during the interactions of bacterial pathogens with their eukaryotic hosts. Other genes required for nodule invasion will also be analyzed, particularly bacA, those function is critically required for nodulation to proceed past the point where the bacteria is surrounded by host membrane and released into the plant cytoplasm. BacA is both highly homologous and isofunctional with the E. coli sbmA gene, whose product is inferred to be a transporter of compounds that contain a (bi)thiazole ring. Experiments will be carried out to determine the mechanism of BacA function. The possible roles of the highly conserved BacA/SbmA homologs identified in the two invasive mammalian pathogens Brucella abortus and Salmonella typhimurium will be assessed. Future work could conceivably lead to the identification of a new class of biologically important (bi)thiazole-containing compounds.